Apparatuses with rotary components, such as gas turbine engines, are subject to vibratory forces at frequencies which are related to the angular velocity of the respective component and hence engine speed. These frequencies are conventionally known as engine order forcing frequencies, each engine order corresponding to a rotational frequency of a particular component (or a fraction or harmonic of the fundamental frequency) and exerting a corresponding vibratory force on the engine.
The forces may arise because e.g. an engine is out of balance on a particular shaft, stiffness irregularities in engine components, and (significantly in the case of gas turbine engines) aerodynamic interactions between the blades of the engine.
At a given engine speed, a number of these engine orders are generally active and result in corresponding vibration responses in the engine which are measurable e.g. as strains or accelerations. Each vibration response generally has the same frequency as the engine order forcing frequency which generated it. However, the relative phase difference between a vibration response and the corresponding engine order may change as the engine speed varies, and particularly when the engine order traverses a resonance frequency of the engine.
Indeed, merely moving toward or away from such a resonance may cause the phase difference to change.
A conventional approach for determining the phase relationship between the forcing frequency of an engine component (e.g. a shaft) and a vibration response is to fit a dedicated once per revolution tachometer to the component. This tachometer would determine the component rotational position (i.e. phase) and also serves as a trigger for the collection of vibration measurements. The approach is illustrated by FIG. 1 which is a flow diagram showing the sequence of data acquisition and analysis events.
The approach is relatively simple in concept, and the synchronisation between the tachometer and the vibration measurements allows the absolute phase difference between the component rotational position and the vibration response to be determined. However, it relies on being able to fit an accurate, robust and dedicated tachometer to the component in question, something which is not always possible for complex components such as the shafts of multi-shaft gas turbine engines. Also the approach precludes deriving simultaneous phase information for other components (e.g. other shafts in a multi-shaft gas turbine), unless the investigator is able to fit further dedicated tachometers which in turn trigger further vibration measurements. Thus, in relation to gas turbine engines, the approach is only generally used for shaft balancing operations, where absolute phase information is needed.
However, U.S. Pat. No. 6,456,945 discloses a method which uses a measurement of absolute phase information to identify an anomaly, such as a crack, in a rotor. In this method, a vibration signal synchronous with the frequency of vibration is filtered from a vibration measurement. A background vibration vector is then subtracted from the vibration signal to produce a vibration difference signal. The phase and amplitude of the vibration difference signal are measured and evaluated to determine whether an anomaly has developed. However, the method is reliant on the rotor turning at a single, fixed trigger speed, and a disadvantage of the method is that a suitable trigger speed has to be known in advance.